Apparatus for detecting abnormality in polishing of a substrate

ABSTRACT

Detection of an abnormality in polishing of a substrate is provided. A measuring device measures a position of the polishing tool relative to a surface of the substrate. A controller determines an amount of polishing of the substrate from the position of the polishing tool; calculates a polishing rate from the amount of polishing of the substrate; and judges that an abnormality in polishing of the edge portion of the substrate has occurred if the polishing rate is out of a predetermined range exceeds a predetermined threshold value.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. application Ser. No. 14/034,488filed on Sep. 23, 2013 which claims priority to Japanese PatentApplication No. 2012-209499 filed Sep. 24, 2012, the entire contents ofwhich are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a method of detecting an abnormality inpolishing of a substrate, such as a wafer, and a polishing apparatus.More particularly, the present invention relates to a method ofdetecting an abnormality in a process of polishing a peripheral portionof a substrate with a polishing tool, such as a polishing tape or afixed abrasive.

Description of the Related Art

A polishing apparatus, which has a polishing tool (e.g., a polishingtape or a fixed abrasive), is used for polishing a peripheral portion ofa wafer. This type of polishing apparatus is configured to bring thepolishing tool into contact with the peripheral portion of the wafer,while rotating the wafer, to thereby polish the peripheral portion. Inthis specification, the peripheral portion of the wafer is defined as aregion including a bevel portion which is the outermost portion of thewafer and a top edge portion and a bottom edge portion located radiallyinwardly of the bevel portion.

FIG. 31A and FIG. 31B are enlarged cross-sectional views each showingthe peripheral portion of the wafer. More specifically, FIG. 31A shows across-sectional view of a so-called straight-type wafer, and FIG. 31Bshows a cross-sectional view of a so-called round-type wafer. In thewafer W shown in FIG. 31A, the bevel portion is an outermostcircumferential surface of the wafer W (indicated by a symbol B) that isconstituted by an upper slope (an upper bevel portion) P, a lower slope(a lower bevel portion) Q, and a side portion (an apex) R. In the waferW shown in FIG. 31B, the bevel portion is a portion B having a curvedcross section and forming an outermost circumferential surface of thewafer W. The top edge portion is a flat portion E1 located radiallyinwardly of the bevel portion B. The bottom edge portion is a flatportion E2 located opposite the top edge portion and located radiallyinwardly of the bevel portion B. The top edge portion E1 and the bottomedge portion E2 may be collectively referred to as edge portion. Theedge portion may include a region where devices are formed.

In a fabrication process of SOI (Silicon on Insulator) substrate, thereis a need to form a vertical surface and a horizontal surface on theedge portion of the wafer W, as shown in FIG. 32. A cross section ofsuch edge portion is achieved by a polishing process as illustrated inFIG. 33. Specifically, while the wafer W is rotated, a right-angle edgeof a polishing tool 400 (e.g., a polishing tape or a fixed abrasive) ispressed against the edge portion of the wafer W to thereby polish it.The polishing tool 400 has its lower surface serving as a polishingsurface that holds abrasive grains thereon. This polishing surface isdisposed in parallel with the wafer W. In this state, the polishingsurface of the polishing tool 400 is pressed against the edge portion ofthe wafer W to thereby form a right-angle cross section, i.e., thevertical surface and the horizontal surface, on the edge portion of thewafer W.

However, the edge portion of the wafer may not be polished properly dueto some causes, such as aged deterioration of the polishing tool 400 ora malfunction of the polishing apparatus. Such an abnormality inpolishing of the wafer would result in a lowered yield. Therefore, it isnecessary to detect the abnormality in polishing of the wafer promptly.However, whether or not the edge portion of the wafer is polishedproperly cannot be judged until the cross section of the edge portion ofthe wafer is examined under a microscope.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method and anapparatus capable of determining whether or not an edge portion of asubstrate, such as a wafer, is polished properly while the substrate isbeing polished.

The inventors have found from wafer polishing results the fact that apolishing rate (i.e., an amount of the wafer polished or removed perunit time) when polishing of the wafer is not performed properly differsfrom a polishing rate when polishing of the wafer is performed properly.

An embodiment is a method of detecting an abnormality in polishing of asubstrate. The method includes: rotating the substrate; pressing apolishing tool against an edge portion of the substrate to polish theedge portion; measuring a position of the polishing tool relative to asurface of the substrate when polishing the edge portion; determining anamount of polishing of the substrate from the position of the polishingtool; calculating a polishing rate from the amount of polishing of thesubstrate; and judging that an abnormality in polishing of the edgeportion of the substrate has occurred if the polishing rate is out of apredetermined range.

Another embodiment is a polishing apparatus for polishing an edgeportion of a substrate. The apparatus includes: a substrate holderconfigured to hold the substrate; a pressing device configured to pressa polishing tool against the edge portion of the substrate; arelative-position measuring device configured to measure a position ofthe polishing tool relative to a surface of the substrate when polishingthe edge portion; and a polishing controller configured to determine anamount of polishing of the substrate from the position of the polishingtool, calculate a polishing rate from the amount of polishing of thesubstrate, and judge that an abnormality in polishing of the edgeportion of the substrate has occurred if the polishing rate is out of apredetermined range.

According to the above embodiments, it is possible to determine whetheror not the edge portion of the substrate is polished properly duringpolishing of the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a polishing apparatus for performing anembodiment of a polishing method;

FIG. 2 is a view illustrating the embodiment of the polishing method;

FIG. 3 shows a cross-sectional view of an edge portion of a polishedwafer and further shows an example of polishing data;

FIG. 4 shows a cross-sectional view of the edge portion of the polishedwafer and further shows another example of polishing data;

FIG. 5 shows a cross-sectional view of the edge portion of the polishedwafer and further shows still another example of polishing data;

FIG. 6 shows a cross-sectional view of the edge portion of the polishedwafer and further shows still another example of polishing data;

FIG. 7 is a view for illustrating a wafer angle shown in FIG. 6;

FIG. 8 is a plan view showing an embodiment of the polishing apparatus;

FIG. 9 is a cross-sectional view taken along line F-F in FIG. 8;

FIG. 10 is a view from a direction indicated by arrow G in FIG. 9;

FIG. 11 is a plan view of a polishing head and a polishing-tape supplyand collection mechanism;

FIG. 12 is a front view of the polishing head and the polishing-tapesupply and collection mechanism;

FIG. 13 is a cross-sectional view taken along line H-H in FIG. 12;

FIG. 14 is a side view of the polishing-tape supply and collectionmechanism shown in FIG. 12;

FIG. 15 is a vertical cross-sectional view of the polishing head asviewed from a direction indicated by arrow I in FIG. 12;

FIG. 16 is a view of a position sensor and a dog as viewed from above;

FIG. 17 is a view of the polishing head and the polishing-tape supplyand collection mechanism that have been moved to predetermined polishingpositions;

FIG. 18 is a schematic view of a pressing pad, a polishing tape, and awafer at the polishing positions as viewed from a lateral direction;

FIG. 19 is a view showing a state in which the pressing pad is pressingthe polishing tape against the wafer;

FIG. 20A, FIG. 20B, and FIG. 20C are views showing operations ofdetecting an edge of the polishing tape;

FIG. 21A is a view of the polishing tape and the pressing pad at thepolishing positions as viewed from a radial direction of the wafer;

FIG. 21B is a view showing a state in which a lower surface of thepressing pad is in contact with an upper surface of the polishing tape;

FIG. 21C is a view showing a state in which the pressing pad is pressingthe polishing tape downwardly against the wafer;

FIG. 22A is a view showing a state in which the wafer is bent as aresult of being pressed by the pressing pad through the polishing tape;

FIG. 22B is a cross-sectional view of the wafer that has been polishedin the state shown in FIG. 22A;

FIG. 23 is a vertical cross-sectional view showing the wafer holderhaving a supporting stage;

FIG. 24 is a perspective view of the supporting stage;

FIG. 25 is a view showing a state in which the holding stage and thewafer held on the upper surface of the holding stage are elevatedrelative to the supporting stage;

FIG. 26 is a view showing an embodiment having a tape stopper;

FIG. 27 is a view showing a state in which the polishing tape isdistorted under a horizontal load;

FIG. 28 is a view showing an embodiment having the tape stopper and atape cover;

FIG. 29 is a view showing an embodiment having a movement-restrictingmechanism for restricting an outward movement of the pressing pad;

FIG. 30 is a view showing an example of a combination of the embodimentshown in FIG. 23 and the embodiment shown in FIG. 29;

FIG. 31A and FIG. 31B are views each showing a peripheral portion of awafer;

FIG. 32 is a view showing a vertical surface and a horizontal surfaceformed on the edge portion of the wafer; and

FIG. 33 is a view illustrating a polishing process for forming thevertical surface and the horizontal surface shown in FIG. 32.

DETAILED DESCRIPTION OF EMBODIMENTS

Embodiments will be described below with reference to the drawings.

FIG. 1 is a schematic view of a polishing apparatus for performing anembodiment of a polishing method. The polishing apparatus includes awafer holder (a substrate holder) 3 configured to hold a wafer W androtate it, a polishing tool 38 configured to polish an edge portion ofthe wafer W, a pneumatic cylinder 53 as a pressing device configured topress the polishing tool 38 against the edge portion of the wafer W, aposition sensor 63 as a relative-position measuring device configured tomeasure a position of the polishing tool 38 relative to a surface of thewafer W, and a polishing controller 11 configured to determine an amountof polishing of the wafer W from a measured value of the position of thepolishing tool 38.

The position sensor 63 is arranged so as to measure the position of thepolishing tool 38 with respect to a direction perpendicular to thesurface of the wafer W. Therefore, an amount of change in the positionof the polishing tool 38 from the surface of the wafer W corresponds tothe amount of polishing of the wafer W. Instead of the position sensor,a displacement sensor may be used as the relative-position measuringdevice. The polishing tool 38 may be a polishing tape, a fixed abrasive,or the like. Instead of the pneumatic cylinder, a combination of aservomotor and a ball screw may be used as the pressing device 53.

FIG. 2 is a view illustrating the embodiment of the polishing method.The polishing tool 38 is arranged such that its polishing surface is inparallel with the surface of the wafer W and the polishing surface facesthe edge portion of the wafer W. Polishing of the wafer W is performedas follows. In step 1, the wafer W is rotated by the wafer holder 3. Instep 2, a gas is supplied into the pneumatic cylinder 53 from a gassupply device (not shown) so that the polishing tool 38 is moved towardthe wafer W until the polishing surface of the polishing tool 38 isbrought into contact with the edge portion of the wafer W. In step 3,the gas is further supplied into the pneumatic cylinder 53 untilpressure of the gas in the pneumatic cylinder 53 reaches a preset value.

In step 4, the position sensor 63 measures the position of the polishingtool 38 for a predetermined initial measurement time while the pressureof the gas in the pneumatic cylinder 53 is kept at the above-describedpreset value. This initial measurement time is a time required formeasured values of the position of the polishing tool 38 to becomestable, and is determined beforehand through previous wafer polishingprocesses and/or experiments. The polishing controller 11 is configuredto calculate an average of the measured values obtained during theinitial measurement time and define this average of the measured valuesas a position reference value. This position reference value is a valueindicating a surface position of the wafer W, and therefore the amountof polishing of the wafer W can be expressed by an amount of movement(or a distance) of the polishing tool 38 from the position referencevalue.

The polishing tool 38 is pressed against the edge portion of the wafer Wby the pneumatic cylinder 53, so that the edge portion of the wafer W ispolished as shown in step 5. The polishing tool 38 has a right-angleedge, which is pressed downwardly against the edge portion of the waferW. The polishing surface of the polishing tool 38 is a horizontalsurface parallel with the wafer surface. This horizontal polishingsurface is pressed against the edge portion of the wafer W to therebyform a vertical surface and a horizontal surface on the edge portion ofthe wafer W.

As the polishing tool 38 polishes the edge portion of the wafer W, theposition of the polishing tool 38 changes. An amount of change in theposition of the polishing tool 38 corresponds to the amount of polishingof the edge portion of the wafer W. Therefore, the polishing controller11 can determine the amount of polishing of the edge portion of thewafer W from the measured value of the position of the polishing tool 38obtained by the position sensor 63. More specifically, a difference(which is an absolute value) between a current measured value and theabove-described position reference value is determined to be the currentamount of polishing. This amount of polishing is expressed as athickness of the wafer W removed by the polishing tool 38, i.e., apolishing depth. The polishing controller 11 terminates polishing of theedge portion of the wafer W by the polishing tool 38 when the determinedamount of polishing reaches a predetermined target amount of polishing.

FIG. 3 through FIG. 6 show cross-sectional views of the edge portion ofthe polished wafer and further show examples of polishing data. In FIG.3 through FIG. 6, a polishing width represents a width of a waferportion polished by the polishing tool 38, and the amount of polishingrepresents an amount (i.e., a depth) of the wafer polished (or removed)by the polishing tool 38, an average polishing rate represents anaverage amount of polishing (or an average polishing depth) per unittime throughout the whole polishing time, a difference represents adifference between the target amount of polishing and an actual amountof polishing, and an error represents a value obtained by dividing thedifference by the target amount of polishing. A wafer angle represents acircumferential angle from a notch (i.e., a cutout) formed on the waferW as shown in FIG. 7. The circumferential angle of the notch is definedas 0 degree.

In FIG. 3 through FIG. 5, the amount of polishing increases atapproximately a constant rate throughout the whole polishing time, andthe vertical surface and the horizontal surface are substantially flat.In contrast, in FIG. 6, the amount of polishing does not increase at aconstant rate throughout the whole polishing time, and the verticalsurface and/or the horizontal surface is not flat. It can be seen fromthese polishing results that, when the amount of polishing increases atapproximately a constant rate throughout the whole polishing time,polishing of the edge portion of the wafer W is performed properly.

Thus, the polishing controller 11 determines, during polishing of thewafer W, the amount of polishing per predetermined time, calculates thepolishing rate from the determined amount of polishing, determineswhether the polishing rate falls within a predetermined range, andjudges that the abnormality in polishing of the wafer W has occurred ifthe polishing rate is out of the predetermined range. The polishingcontroller 11 may judge that the abnormality in polishing of the wafer Whas occurred if the number of times the polishing rate goes beyond thepredetermined range reaches a predetermined number. Alternatively, thepolishing controller 11 may judge that the abnormality in polishing ofthe wafer W has occurred if a period of time the polishing rate is outof the predetermined range exceeds a predetermined threshold value.

In the example shown in FIG. 5, the polishing rate is approximatelyconstant throughout the entire polishing time and the polishing ratefalls within the predetermined range. Therefore, the polishingcontroller 11 judges that polishing of the edge portion of the wafer Wis performed properly. In contrast, in the example shown in FIG. 6, thepolishing rate is not constant throughout the entire polishing time andthe polishing rate is out of the predetermined range in some timepoints. Specifically, the polishing rate is approximately 0 at a pointof time when 150 seconds have passed, while the polishing rate increasesgreatly at a point of time when 380 seconds have passed. Therefore, inthis case, the polishing controller 11 judges that polishing of the edgeportion of the wafer W is not performed properly.

In this manner, the polishing controller 11 can determine the occurrenceof the abnormality in polishing of the wafer W while the wafer W isbeing polished. Therefore, the polishing controller 11 can performreal-time monitoring of the polishing of the edge portion of the waferW. Examples of the abnormality in polishing of the wafer W include acase where a linearity showing a relationship between the amount ofpolishing and the polishing time is lost and a case where a totalpolishing rate throughout the entire polishing time is too low or toohigh.

As described previously, the amount of polishing can be determined fromthe measured value of the position of the polishing tool 38 obtainedfrom the position sensor 63. In order to obtain the amount of polishingmore accurately, the polishing controller 11 may calculate a movingaverage of the measured values sent from the position sensor 63 and maydetermine the amount of polishing from the moving average. Thisoperation can eliminate an influence of a variation in the measuredvalue of the position of the polishing tool 38.

When the abnormality in the wafer polishing is detected, the polishingcontroller 11 may terminate the polishing of the edge portion of thewafer W, and further may generate an alarm signal and transmit it to theexterior of the polishing controller 11. Further, when the abnormalityin the wafer polishing is detected, the polishing controller 11 maychange polishing conditions for subsequent wafers. For example, if thecalculated polishing rate is smaller than the predetermined range, thenthe pressure applied from the polishing tool 38 to the subsequent wafersmay be increased and/or the rotational speed of the subsequent wafersmay be increased. In this manner, it is possible to prevent theoccurrence of the abnormality in polishing of the subsequent wafers byreflecting the polishing rate, which has been calculated duringpolishing of the wafer W, on the polishing conditions for the subsequentwafers.

Next, the details of the polishing apparatus will be described. FIG. 8is a plan view showing a polishing apparatus according to an embodiment,FIG. 9 is a cross-sectional view taken along line F-F in FIG. 8, andFIG. 10 is a view from a direction indicated by arrow G in FIG. 9. Thepolishing apparatus according to the embodiment includes the waferholder (substrate holder) 3 configured to hold the wafer (substrate) W(i.e., a workpiece to be polished) horizontally and to rotate the waferW. FIG. 8 shows a state in which the wafer holder 3 holds the wafer W.This wafer holder 3 has a holding stage 4 configured to hold a lowersurface of the wafer W by a vacuum suction, a hollow shaft 5 coupled toa central portion of the holding stage 4, and a motor M1 for rotatingthe hollow shaft 5. The wafer W is placed onto the holding stage 4 suchthat a center of the wafer W is aligned with a central axis of thehollow shaft 5.

The holding stage 4 is located in a polishing chamber 22 that is definedby a partition 20 and a base plate 21. The partition 20 has an entrance20 a through which the wafer W is transported into and removed from thepolishing chamber 22. This entrance 20 a is in the shape of a horizontalcutout and can be closed with a shutter 23.

The hollow shaft 5 is supported by ball spline bearings (i.e., linearmotion bearings) 6 which allow the hollow shaft 5 to move vertically.The holding stage 4 has an upper surface with grooves 4 a. These grooves4 a communicate with a communication passage 7 extending through thehollow shaft 5. The communication passage 7 is coupled to a vacuum line9 via a rotary joint 8 provided on a lower end of the hollow shaft 5.The communication passage 7 is also coupled to a nitrogen-gas supplyline 10 for use in releasing the wafer W from the holding stage 4 afterprocessing. By selectively coupling the vacuum line 9 and thenitrogen-gas supply line 10 to the communication passage 7, the wafer Wcan be held on the upper surface of the holding stage 4 by the vacuumsuction and can be released from the upper surface of the holding stage4.

A pulley p1 is coupled to the hollow shaft 5, and a pulley p2 is mountedto a rotational shaft of the motor M1. The hollow shaft 5 is rotated bythe motor M1 through the pulley p1, the pulley p2, and a belt b1 ridingon these pulleys p1 and p2. The ball spline bearing 6 is a bearing thatallows the hollow shaft 5 to move freely in its longitudinal direction.The ball spline bearings 6 are secured to a cylindrical casing 12.Therefore, the hollow shaft 5 can move linearly up and down relative tothe casing 12, and the hollow shaft 5 and the casing 12 rotate inunison. The hollow shaft 5 is coupled to a pneumatic cylinder (elevatingmechanism) 15, so that the hollow shaft 5 and the holding stage 4 areelevated and lowered by the pneumatic cylinder 15.

A cylindrical casing 14 is provided so as to surround the casing 12 in acoaxial arrangement. Radial bearings 18 are provided between the casing12 and the casing 14, so that the casing 12 is rotatably supported bythe radial bearings 18. With these structures, the wafer holder 3 canrotate the wafer W about its central axis and can elevate and lower thewafer W along its central axis.

A polishing unit 25 for polishing a peripheral portion of the wafer W isprovided radially outwardly of the wafer W held by the wafer holder 3.This polishing unit 25 is located in the polishing chamber 22. As shownin FIG. 10, the polishing unit 25 in its entirety is secured to a mountbase 27, which is coupled to a polishing-unit moving mechanism 30 via anarm block 28.

The polishing-unit moving mechanism 30 has a ball screw mechanism 31that holds the arm block 28, a motor 32 for driving the ball screwmechanism 31, and a power transmission mechanism 33 that couples theball screw mechanism 31 and the motor 32 to each other. The powertransmission mechanism 33 is constructed by pulleys, a belt, and thelike. As the motor 32 operates, the ball screw mechanism 31 moves thearm block 28 in directions indicated by arrows in FIG. 10 to therebymove the polishing unit 25 in its entirety in a tangential direction ofthe wafer W. This polishing-unit moving mechanism 30 also serves as anoscillation mechanism for oscillating the polishing unit 25 at apredetermined amplitude and a predetermined speed.

The polishing unit 25 includes a polishing head 50 for polishing theperiphery of the wafer W using a polishing tape 38, and a polishing-tapesupply and collection mechanism 70 for supplying the polishing tape 38to the polishing head 50 and collecting the polishing tape 38 from thepolishing head 50. The polishing head 50 is a top-edge polishing headfor polishing the top edge portion of the wafer W by pressing apolishing surface of the polishing tape 38 downwardly against theperipheral portion of the wafer W. The polishing-tape supply andcollection mechanism 70 also serves as a polishing-tape supportingmechanism for supporting the polishing tape 38 in parallel with thesurface of the wafer W.

FIG. 11 is a plan view of the polishing head 50 and the polishing-tapesupply and collection mechanism 70, FIG. 12 is a front view of thepolishing head 50 and the polishing-tape supply and collection mechanism70, FIG. 13 is a cross-sectional view taken along line H-H in FIG. 12,FIG. 14 is a side view of the polishing-tape supply and collectionmechanism 70 shown in FIG. 12, and FIG. 15 is a vertical cross-sectionalview of the polishing head 50 as viewed from a direction indicated byarrow I in FIG. 12.

Two linear motion guides 40A and 40B, which extend parallel to theradial direction of the wafer W, are disposed on the mount base 27. Thepolishing head 50 and the linear motion guide 40A are coupled to eachother via a coupling block 41A. Further, the polishing head 50 iscoupled to a motor 42A and a ball screw 43A for moving the polishinghead 50 along the linear motion guide 40A (i.e., in the radial directionof the wafer W). More specifically, the ball screw 43A is secured to thecoupling block 41A, and the motor 42A is secured to the mount base 27through a support member 44A. The motor 42A is configured to rotate ascrew shaft of the ball screw 43A, so that the coupling block 41A andthe polishing head 50 (which is coupled to the coupling block 41A) aremoved along the linear motion guide 40A. The motor 42A, the ball screw43A, and the linear motion guide 40A constitute a first moving mechanism45 for moving the polishing head 50 in the radial direction of the waferW held on the wafer holder 3.

Similarly, the polishing-tape supply and collection mechanism 70 and thelinear motion guide 40B are coupled to each other via a coupling block41B. Further, the polishing-tape supply and collection mechanism 70 iscoupled to a motor 42B and a ball screw 43B for moving thepolishing-tape supply and collection mechanism 70 along the linearmotion guide 40B (i.e., in the radial direction of the wafer W). Morespecifically, the ball screw 43B is secured to the coupling block 41B,and the motor 42B is secured to the mount base 27 through a supportmember 44B. The motor 42B is configured to rotate a screw shaft of theball screw 43B, so that the coupling block 41B and the polishing-tapesupply and collection mechanism 70 (which is coupled to the couplingblock 41B) are moved along the linear motion guide 40B. The motor 42B,the ball screw 43B, and the linear motion guide 40B constitute a tapemoving mechanism (a second moving mechanism) 46 for moving the polishingtape 38 and the polishing-tape supply and collection mechanism (i.e.,the polishing-tape supporting mechanism) 70 in the radial direction ofthe wafer W held on the wafer holder 3.

As shown in FIG. 15, the polishing head 50 has the pressing pad 51 forpressing the polishing tape 38 against the wafer W, a pad holder 52 thatholds the pressing pad 51, and the pneumatic cylinder 53 as the pressingdevice configured to push down the pad holder 52 (and the pressing pad51). The pneumatic cylinder 53 is held by a holding member 55. Further,the holding member 55 is coupled to a pneumatic cylinder 56 serving as alifter via a linear motion guide 54 extending in a vertical direction.As a gas (e.g., air) is supplied to the pneumatic cylinder 56 from anon-illustrated gas supply source, the pneumatic cylinder 56 pushes upthe holding member 55, whereby the holding member 55, the pneumaticcylinder 53, the pad holder 52, and the pressing pad 51 are elevatedalong the linear motion guide 54.

In this embodiment, the vertically-moving mechanism 59 for moving thepressing pad 51 in the direction perpendicular to the wafer surface isconstituted by the pneumatic cylinder 53 and the pneumatic cylinder 56.The motor 42A, the ball screw 43A, and the linear motion guide 40Aconstitute the first moving mechanism 45, which also serves as aradially-moving mechanism for moving the pressing pad 51 and thevertically-moving mechanism 59 in the radial direction of the wafer W.Further, the polishing-unit moving mechanism 30 serves as atangentially-moving mechanism for moving the pressing pad 51 (and thepressing device 53) in the tangential direction of the wafer W.

The pneumatic cylinder 56 is secured to a mount member 57 that is fixedto the coupling block 41A. The mount member 57 and the pad holder 52 arecoupled to each other via a linear motion guide 58 extending in thevertical direction. When the pad holder 52 is pushed down by thepneumatic cylinder 53, the pressing pad 51 is moved downward along thelinear motion guide 58 to thereby press the polishing tape 38 againstthe peripheral portion of the wafer W. The pressing pad 51 is made ofresin (e.g., PEEK (polyetheretherketone)), metal (e.g., stainlesssteel), or ceramic (e.g., SiC (silicon carbide)).

The pressing pad 51 has through-holes 51 a extending in the verticaldirection. A vacuum line 60 is coupled to the through-holes 51 a. Thisvacuum line 60 has a valve (not shown in the drawings) therein. Byopening this valve, a vacuum is produced in the through-holes 51 a ofthe pressing pad 51. When the vacuum is produced in the through-holes 51a with the pressing pad 51 in contact with an upper surface of thepolishing tape 38, this upper surface of the polishing tape 38 is heldon a lower surface of the pressing pad 51. Only one through-hole 51 amay be provided in the pressing pad 51.

The pad holder 52, the pneumatic cylinder 53, the holding member 55, thepneumatic cylinder 56, and the mount member 57 are housed in a box 62. Alower portion of the pad holder 52 projects from a bottom of the box 62,and the pressing pad 51 is attached to this lower portion of the padholder 52. The position sensor 63 for detecting a vertical position ofthe pressing pad 51 is disposed in the box 62. This position sensor 63is mounted to the mount member 57. A dog 64, which serves as a sensortarget, is provided on the pad holder 52. The position sensor 63 isconfigured to detect the vertical position of the pressing pad 51 basedon the vertical position of the dog 64.

FIG. 16 is a view of the position sensor 63 and the dog 64 as viewedfrom above. The position sensor 63 has a light emitter 63A and a lightreceiver 63B. When the dog 64 is lowered together with the pad holder 52(and the pressing pad 51), a part of light emitted from the lightemitter 63A is interrupted by the dog 64. Therefore, the position of thedog 64, i.e., the vertical position of the pressing pad 51, can bedetected from a quantity of the light received by the light receiver63B. The position sensor 63 shown in FIG. 16 is a so-called transmissionoptical sensor. However, other type of position sensor may be used.

The polishing-tape supply and collection mechanism 70 has a supply reel71 for supplying the polishing tape 38 and a collection reel 72 forcollecting the polishing tape 38. The supply reel 71 and the collectionreel 72 are coupled to tension motors 73 and 74, respectively. Thesetension motors 73 and 74 are configured to apply predetermined torque tothe supply reel 71 and the collection reel 72 to thereby exert apredetermined tension on the polishing tape 38.

A polishing-tape advancing mechanism 76 is provided between the supplyreel 71 and the collection reel 72. This polishing-tape advancingmechanism 76 has a tape-advancing roller 77 for advancing the polishingtape 38, a nip roller 78 that presses the polishing tape 38 against thetape-advancing roller 77, and a tape-advancing motor 79 for rotating thetape-advancing roller 77. The polishing tape 38 is interposed betweenthe tape-advancing roller 77 and the nip roller 78. By rotating thetape-advancing roller 77 in a direction indicated by arrow in FIG. 12,the polishing tape 38 is advanced from the supply reel 71 to thecollection reel 72.

The tension motors 73 and 74 and the tape-advancing motor 79 are mountedto a pedestal 81. This pedestal 81 is secured to the coupling block 41B.The pedestal 81 has two support arms 82 and 83 extending from the supplyreel 71 and the collection reel 72 toward the polishing head 50. Aplurality of guide rollers 84A, 84B, 84C, 84D, and 84E for supportingthe polishing tape 38 are provided on the support arms 82 and 83. Thepolishing tape 38 is guided by these guide rollers 84A to 84E so as tosurround the polishing head 50.

The extending direction of the polishing tape 38 is perpendicular to theradial direction of the wafer W as viewed from above. The two guiderollers 84D and 84E, which are located below the polishing head 50,support the polishing tape 38 such that the polishing surface of thepolishing tape 38 is parallel to the surface (upper surface) of thewafer W. Further, the polishing tape 38 extending between these guiderollers 84D and 84E is parallel to the tangential direction of the waferW. There is a gap in the vertical direction between the polishing tape38 and the wafer W.

The polishing apparatus further has a tape-edge detection sensor 100 fordetecting a position of the edge of the polishing tape 38. Thistape-edge detection sensor 100 is a transmission optical sensor, as wellas the above-described position sensor 63. The tape-edge detectionsensor 100 has a light emitter 100A and a light receiver 100B. The lightemitter 100A is secured to the mount base 27 as shown in FIG. 11, andthe light receiver 100B is secured to the base plate 21 that defines thepolishing chamber 22 as shown in FIG. 9. This tape-edge detection sensor100 is configured to detect the position of the edge of the polishingtape 38 based on a quantity of the light received by the light receiver100B.

As shown in FIG. 17, when polishing the wafer W, the polishing head 50and the polishing-tape supply and collection mechanism 70 are moved totheir predetermined polishing positions, respectively, by the motors 42Aand 42B and the ball screws 43A and 43B. The polishing tape 38 at thepolishing position extends in the tangential direction of the wafer W asviewed from above the wafer W. Therefore, the polishing-tape supply andcollection mechanism 70 serves as a polishing-tape supporting mechanismfor supporting the polishing tape 38 in parallel with the surface of thewafer W such that the polishing tape 38 extends along the tangentialdirection of the wafer W.

FIG. 18 is a schematic view of the polishing pad 51, the polishing tape38, and the wafer W at the polishing positions as viewed from thelateral direction. As shown in FIG. 18, the polishing tape 38 is locatedabove the edge portion of the wafer W, and the pressing pad 51 islocated above the polishing tape 38. The edge of the pressing pad 51 andthe edge of the polishing tape 38 at their polishing positions coincidewith each other. That is, the polishing head 50 and the polishing-tapesupply and collection mechanism 70 are moved independently to theirrespective polishing positions such that the edge of the pressing pad 51and the edge of the polishing tape 38 coincide with each other. FIG. 19is a view showing a state in which the pressing pad 51 is pressing thepolishing tape 38 against the wafer W.

The polishing tape 38 is a long and narrow strip-shaped polishing tool.Although a width of the polishing tape 38 is basically constantthroughout its entire length, there may be a slight variation in thewidth of the polishing tape 38 in some parts thereof. As a result, theposition of the edge of the polishing tape 38 at its polishing positionmay vary from wafer to wafer. On the other hand, the position of thepressing pad 51 at its polishing position is constant at all times.Thus, in order to enable the edge of the polishing tape 38 to coincidewith the edge of the pressing pad 51, the position of the edge of thepolishing tape 38 is detected by the above-described tape-edge detectionsensor 100 before the polishing tape 38 is moved to its polishingposition.

FIG. 20A through FIG. 20C are views illustrating operations fordetecting the edge of the polishing tape 38. Prior to polishing of thewafer W, the polishing tape 38 is moved from a retreat position shown inFIG. 20A to a tape-edge detecting position shown in FIG. 20B. In thistape-edge detecting position, the position of the wafer-side edge of thepolishing tape 38 is detected by the tape-edge detection sensor 100.Then, as shown in FIG. 20C, the polishing tape 38 is moved to thepolishing position such that the edge of the polishing tape 38 coincideswith the edge of the pressing pad 51. Because the polishing tape 38 ismovable independently of the polishing head 50, the polishing tape 38can be moved by a distance that can vary depending on the width of thepolishing tape 38.

The position of the edge of the pressing pad 51 at the polishingposition is stored in advance in the polishing controller 11 (see FIG.8). Therefore, the polishing controller 11 can calculate the traveldistance of the polishing tape 38 for allowing the edge of the polishingtape 38 to coincide with the edge of the pressing pad 51 from thedetected edge position of the polishing tape 38 and the edge position ofthe pressing pad 51. In this manner, the travel distance of thepolishing tape 38 is determined based on the detected position of theedge of the polishing tape 38. Therefore, the edge of the polishing tape38 can be aligned with the edge of the pressing pad 51 regardless of avariation in the width of the polishing tape 38.

Next, polishing operations of the polishing apparatus having theabove-described structures will be described. The following operationsof the polishing apparatus are controlled by the polishing controller 11shown in FIG. 8. The wafer W is held by the wafer holder 3 such that afilm (e.g., a device layer) formed on the surface thereof faces upward,and further the wafer W is rotated about its center. Liquid (e.g., purewater) is supplied to the center of the rotating wafer W from a liquidsupply nozzle (not shown in the drawings). The pressing pad 51 of thepolishing head 50 and the polishing tape 38 are moved to thepredetermined polishing positions, respectively, as shown in FIG. 18.

FIG. 21A is a view of the polishing tape 38 and the pressing pad 51 atthe polishing positions as viewed from the radial direction of the waferW. The pressing pad 51 shown in FIG. 21A is in an upper position as aresult of being elevated by the pneumatic cylinder 56 (see FIG. 15). Inthis position, the pressing pad 51 is located above the polishing tape38. Subsequently, the operation of the pneumatic cylinder 56 is stoppedand as a result a piston rod thereof is lowered. The pressing pad 51 islowered until its lower surface contacts the upper surface of thepolishing tape 38 as shown in FIG. 21B. In this state, the vacuum isproduced in the through-holes 51 a of the pressing pad 51 through thevacuum line 60 to enable the lower surface of the pressing pad 51 tohold the polishing tape 38. While holding the polishing tape 38, thepressing pad 51 is lowered by the pneumatic cylinder 53 (see FIG. 15) topress the polishing surface of the polishing tape 38 against theperipheral portion of the wafer W at a predetermined polishing pressure,as shown in FIG. 21C. The polishing pressure is a pressure applied fromthe polishing tape 38 to the edge portion of the wafer W. The polishingpressure can be adjusted by the pressure of the gas supplied to thepneumatic cylinder 53.

The edge portion of the wafer W is polished by the sliding contactbetween the rotating wafer W and the polishing tape 38. In order toincrease the polishing rate of the wafer W, the polishing tape 38 andthe pressing pad 51 may be oscillated in the tangential direction of thewafer W by the polishing-unit moving mechanism (tangentially-movingmechanism) 30 during polishing of the wafer W. During polishing, theliquid (e.g., pure water) is supplied onto the center of the rotatingwafer W, so that the wafer W is polished in the presence of the water.The liquid, supplied to the wafer W, spreads over the upper surface ofthe wafer W in its entirety via a centrifugal force. This liquid canprevent polishing debris from contacting devices of the wafer W. Asdescribed above, during polishing, the polishing tape 38 is held on thepressing pad 51 by the vacuum suction. Therefore, a relative change inposition between the polishing tape 38 and the pressing pad 51 isprevented. As a result, a polishing position and a polishing profile canbe stable. Further, even when the polishing pressure is increased, therelative position between the polishing tape 38 and the pressing pad 51does not change. Therefore, a polishing time can be shortened.

The vertical position of the pressing pad 51 during polishing of thewafer W is detected by the position sensor 63. Therefore, a polishingend point can be detected from the vertical position of the pressing pad51. For example, polishing of the edge portion of the wafer W can beterminated when the vertical position of the pressing pad 51 has reacheda predetermined target position. This target position is determinedaccording to a target amount of polishing.

As shown in the step 2 through the step 5 in FIG. 2, the position of thepolishing tape 38 relative to the surface of the wafer W can bedetermined from the vertical position of the pressing pad 51. The amountof change in the position of the polishing tape 38 from the surface ofthe wafer W corresponds to the amount of polishing of the wafer W. Themeasured value outputted from the position sensor 63 is sent to thepolishing controller 11, which determines the position reference valuefrom the measured value of the position of the pressing pad 51 andfurther determines the amount of polishing of the edge portion of thewafer W, as discussed with reference to FIG. 2. Instead of the positionsensor 63, a displacement sensor may be used.

As discussed previously, the polishing controller 11 calculates thepolishing rate during polishing of the wafer W, determines whether ornot the calculated polishing rate falls within the predetermined range,and judges that the abnormality in polishing of the wafer W has occurredif the number of times the polishing rate goes beyond the predeterminedrange reaches a predetermined number.

When polishing of the wafer W is terminated, supply of the gas to thepneumatic cylinder 53 is stopped, whereby the pressing pad 51 iselevated to the position shown in FIG. 21B. At the same time, the vacuumsuction of the polishing tape 38 is stopped. Further, the pressing pad51 is elevated by the pneumatic cylinder 56 to the position shown inFIG. 21A. The polishing head 50 and the polishing-tape supply andcollection mechanism 70 are moved to the retreat positions shown in FIG.11. The polished wafer W is elevated by the wafer holder 3 andtransported to the exterior of the polishing chamber 22 by hands of anon-illustrated transporting mechanism. Before polishing of the nextwafer is started, the polishing tape 38 is advanced from the supply reel71 to the collection reel 72 by a predetermined distance by thetape-advancing mechanism 76, so that a new polishing surface is used forpolishing of the next wafer. When the polishing tape 38 is estimated tobe clogged with the polishing debris, the polished wafer W may bepolished again with the new polishing surface after the polishing tape38 is advanced by the predetermined distance. Clogging of the polishingtape 38 can be estimated from, for example, the polishing time and thepolishing pressure. Polishing of the wafer W may be performed whileadvancing the polishing tape 38 at a predetermined speed by thetape-advancing mechanism 76. It is possible to advance the polishingtape 38 in its longitudinal direction by the polishing-tape advancingmechanism 76 while holding the polishing tape 38 on the pressing pad 51by the vacuum suction. The polishing tape 38 may not be held on thepressing pad 51 by the vacuum suction.

As shown in FIG. 22A, when an increased polishing pressure is exerted onthe wafer W, the wafer W is greatly bent by the polishing pressure ofthe pressing pad 51, and as a result, an oblique polished surface isformed on the wafer W as shown in FIG. 22B. Thus, in an embodiment shownin FIG. 23 a supporting stage 180 for supporting a lower surface of theperipheral portion of the wafer W is provided in the wafer holder 3. Thesame parts as those shown in FIG. 9 will not be described belowrepetitively. The supporting stage 180 is fixed to a supporting stagebase 181. This supporting stage base 181 is fixed to the upper end ofthe casing 12 and rotates in unison with the casing 12. Accordingly, thesupporting stage 180 rotates in unison with the casing 12 and theholding stage 4.

The supporting stage 180 has an inverted truncated cone shape as shownin FIG. 24 for supporting the lower surface of the peripheral portion ofthe wafer W in its entirety. The lower surface of the peripheral portionof the wafer W supported by the supporting stage 180 is a regionincluding at least the bottom edge portion E2 shown in FIG. 30A and FIG.30B. The supporting stage 180 has an annular upper surface 180 a thatprovides a supporting surface for supporting the lower surface of theperipheral portion of the wafer W. When the wafer W is polished, anoutermost edge of the supporting stage 180 and an outermost edge of thewafer W approximately coincide with each other.

Use of such supporting stage 180 can prevent the wafer W from being bentwhen the pressing pad 51 presses the polishing tape 38 against the waferW. Therefore, the polishing tape 38 can polish the edge portion of thewafer W to form the vertical surface and the horizontal surface on theedge portion of the wafer W. Because the supporting stage 180 supportsthe lower surface of the peripheral portion of the wafer W in itsentirety, the polishing tape 38 can polish the peripheral portion of thewafer W uniformly, compared with a case of using a conventional wafersupporting mechanism that supports only a part of the wafer.

The ball spline bearings 6 are disposed between the hollow shaft 5 andthe casing 12, so that the hollow shaft 5 can move up and down relativeto the casing 12. Therefore, the holding stage 4 coupled to an upper endof the hollow shaft 5 can move up and down relative to the casing 12 andthe supporting stage 180. FIG. 25 shows a state in which the holdingstage 4 and the wafer W held on the upper surface of the holding stage 4are elevated relative to the supporting stage 180.

The polishing tape 38 may receive a horizontal load due to contact withthe wafer W or an influence of the shape of the peripheral portion ofthe wafer W. As a result, the polishing tape 38 may be forced to moveoutwardly of the wafer W. Thus, a tape stopper 185 for restricting ahorizontal movement of the polishing tape 38 is provided on the pressingpad 51 as shown in FIG. 26. The tape stopper 185 is arranged outwardlyof the polishing tape 38 with respect to the radial direction of thewafer W so as to restrict an outward movement of the polishing tape 38.This tape stopper 185 can prevent the polishing tape 38 from movingoutwardly of the wafer W. Therefore, a polishing profile and a polishingwidth of the wafer W can be stable.

When the tape stopper 185 receives the outward movement of the polishingtape 38, the polishing tape 38 may be distorted as shown in FIG. 27.Thus, in an embodiment shown in FIG. 28, in order to prevent thedistortion of the polishing tape 38, a tape cover 186 is provided inproximity to the polishing surface of the polishing tape 38. The tapecover 186 is secured to the tape stopper 185 and is arranged so as tocover a large part of the polishing surface of the polishing tape 38.The tape cover 186 is located below the polishing tape 38 such that asmall gap dg is formed between the polishing surface of the polishingtape 38 and an upper surface of the tape cover 186. The polishing tape38 is arranged between the pressing pad 51 and the tape cover 186. Byproviding such tape cover 186, the polishing tape 38 can be preventedfrom being distorted and can be kept flat. Therefore, the polishingprofile and the polishing width of the wafer W can be stable.

As shown in FIG. 28, the polishing tape 38 is located in a spacesurrounded by the pressing pad 51, the tape stopper 185, and the tapecover 186. A gap h between the lower surface of the pressing pad 51 andthe upper surface of the tape cover 186 is larger than a thickness ofthe polishing tape 38. A gap dg between the polishing tape 38 and thetape cover 186 is smaller than a thickness of the wafer W.

The tape cover 186 has an inner side surface 186 a located outwardly ofthe edge 51 b of the pressing pad 51 with respect to the radialdirection of the wafer W. Therefore, the polishing surface of thepolishing tape 38 is exposed by a distance dw between the edge of thepolishing tape 38 and the inner side surface 186 a of the tape cover186. Polishing of the wafer W is performed by this exposed polishingsurface.

In the structures shown in FIG. 28, the tape stopper 185 receives thehorizontal load acting on the polishing tape 38. As a result, thepressing pad 51 may move outwardly together with the polishing tape 38.Such a movement of the pressing pad 51 destabilizes the polishingprofile and the polishing width. Thus, in an embodiment shown in FIG.29, a movement-restricting mechanism for restricting the outwardmovement of the pressing pad 51 is provided. This movement-restrictingmechanism has a projecting member 190 fixed to the pressing pad 51 andfurther has a side stopper 191 for restricting a horizontal movement ofthis projecting member 190. In this embodiment, a plunger is used as theprojecting member 190.

The side stopper 191 is disposed outwardly of the plunger (projectingmember) 190 with respect to the radial direction of the wafer W so as toreceive an outward movement of the plunger 190. The side stopper 191 issecured to the lower surface of the box 62 of the polishing head 50, sothat a position of the side stopper 191 is fixed. The plunger 190 andthe side stopper 191 are arranged in proximity to each other, and a gapdr between the plunger 190 and the side stopper 191 is in a range of 10μm to 100 μm. With this structure, when the pressing pad 51 movesoutwardly upon receiving the horizontal load from the polishing tape 38during polishing, the plunger 190 is brought into contact with the sidestopper 191, whereby the outward movements of the pressing pad 51 andthe polishing tape 38 are restricted. Therefore, the polishing profileand the polishing width of the wafer W can be stable.

The embodiments shown in FIG. 23 through FIG. 29 can be combined in anappropriate manner. For example, FIG. 30 shows an example in which thesupporting stage 180 shown in FIG. 23 and the polishing head 50 shown inFIG. 29 are combined. This structure shown in FIG. 30 can prevent thedeflection of the wafer W and can further prevent the movement and thedistortion of the polishing tape 38.

While the above-discussed embodiments of the polishing apparatus use thepolishing tape as the polishing tool, the present invention is notlimited to such embodiments and can be applied to other polishingapparatus using a fixed abrasive (a grinding stone) as the polishingtool.

The previous description of embodiments is provided to enable a personskilled in the art to make and use the present invention. Moreover,various modifications to these embodiments will be readily apparent tothose skilled in the art, and the generic principles and specificexamples defined herein may be applied to other embodiments. Therefore,the present invention is not intended to be limited to the embodimentsdescribed herein but is to be accorded the widest scope as defined bylimitation of the claims and equivalents.

What is claimed is:
 1. An apparatus for detecting an abnormality inpolishing of a substrate, said apparatus comprising: a substrate holderconfigured to rotate the substrate; a pressing device configured topress a polishing tool against an edge portion of the substrate topolish the edge portion; a measuring device configured to measure aposition of the polishing tool relative to a surface of the substrate;and a controller configured to determine an amount of polishing of thesubstrate from the position of the polishing tool, calculate a polishingrate from the amount of polishing of the substrate, and judge whether ornot an abnormality in polishing of the edge portion of the substrate hasoccurred, the abnormality occurring if a period of time the polishingrate is out of a predetermined range exceeds a predetermined thresholdvalue.
 2. The apparatus according to claim 1, wherein the controller isconfigured to terminate polishing of the edge portion of the substrateif the abnormality in polishing of the edge portion of the substrate isjudged to have occurred.
 3. The apparatus according to claim 1, whereinthe controller is configured to generate an alarm if the abnormality inpolishing of the edge portion of the substrate is judged to haveoccurred.
 4. The apparatus according to claim 1, wherein the polishingtool is a polishing tape.
 5. The apparatus according to claim 1, whereinthe polishing tool is a fixed abrasive.
 6. The apparatus according toclaim 1, wherein the pressing device is configured to increase apressure to be applied from the polishing tool to a subsequent substrateif the polishing rate is lower than the predetermined range.
 7. Theapparatus according to claim 1, wherein the substrate holder isconfigured to increase a rotational speed of a subsequent substrate ifthe polishing rate is lower than the predetermined range.